The present invention is directed to a method of making stable aqueous emulsions of organopolysiloxane particles having a volume average particle diameter (Dv) of at least about 300 nanometers (nm) which have been functionalized with a chemically combined polar group such as a mercaptoorgano, epoxyorgano, or carboxy group.
There is shown by Craig in U.S. Pat. Nos. 5,726,270, 5,856,402, and 5,900,460, semi-continuous methods for making acid catalyzed aqueous dispersions of organopolysiloxane particles from cyclic poly(diorganosiloxane)s. The aqueous dispersions of organopolysiloxane particles have been found to be useful in coating applications including personal care. For example, in the formulation of a silicone composition useful in hair shampoos, experience has shown that desirable results can be achieved with the use of an aqueous dispersion of organopolysiloxane particles having a volume average particle diameter in the range between about 300 nanometers and about 1 micron. It has been found that improvements in hair styling can be obtained with a silicone-containing shampoo, because the silicone can impart a "smoothing out" effect. It is speculated that shampoos containing silicones in the form of aqueous dispersions of organopolysiloxane particles having a volume average particle diameter (Dv) of at least about 300 nanometers, can more readily coat the surface of the hair shaft to achieve a smoother surface.
It also has been suggested that additional enhancements can be obtained in coating and personal care applications based on the use of aqueous dispersions of organopolysiloxane particles by modifying the dispersed particles with a polar functional group, such as a mercaptoorgano, epoxyorgano, or carboxy radical. However, previous efforts to synthesize aqueous emulsions of functionalized organopolysiloxane particles having a volume average particle diameter of at least about 300 nanometers in a semi-continuous manner have been unsuccessful. Attempts to incorporate an organosilicon polar functionalizing source material, for example an alkoxy silane having a polar functional group attached to silicon by a carbon-silicon bond, into an aqueous organopolysiloxane equilibration mixture of cyclic poly(diorganosiloxane) using a conventional acid catalyst typically results in organopolysiloxane particles with volume average particle diameter less than 300 nanometers. It is speculated that the limitation in particle size growth in such a polar group-substituted alkoxysilane-containing mixture is due to a difference in reactivity between the cyclic poly(diorganosiloxane) and alkoxysilane in an acid catalyzed environment. A satisfactory method is therefor needed to make stable aqueous emulsions of functionalized organopolysiloxane particles having a volume average particle diameter of at least about 300 nanometers using an acid catalyzed process.